Current converting tube



CURRENT CONVERTING TUBE Filed Feb. 27, 1947 2 Sheets-Sheet 1 ATTOR/VE Y Nov. 2, 1948. w, D R 2,452,861

CURRENT CONVERTING TUBE Filed Feb. 27, .1947 2 sneaks-sheet 2 Q fizz:

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Patented Nov. 2, 1948 CURRENT CONVERTING TUBE Johannes Gijsbertus Wilhelm Mulder, Eindhoven, Netherlands, assignor to Hartford National Bank and Trust Company, Hartford, Conn., as

trustee Application February 27, 1947, Serial No. 731,289

In the Netherland s October 1'7, 1941 Section 1, Public Law 690, August 8, 1946 Patent expires October 1'7, 1961 4 Claims. (01. 250-215) This invention relates to a current converting tube which in a common vessel contains an oxide cathode, one or more anodes and a gas and/or vapour filling having a pressure of more than 0.3 mm. of mercury at operating temperatures and in which the effective anode supply voltage exceeds 100 volts and the rectified current is at least amps.

With the current converting tubes hereinbefore described experience has revealed that frequently without any ascertainable effect being responsible for it the tubes exhibit back-fires. Particularly in those cases in which a stabilising impedance is not connected in series with the anodes of the tube and the source of anode voltage, this phenomenon may lead to serious operating disturbance and even to deterioration of the tube.

In view of the foregoing it has always been the object of the current converting tube industry to choose the constructionof the tubes to be such that in the case of higher operating voltages and currents screenings are arranged between anode and cathode. A method in use for this purpose is the withdrawal of the anode into a separate arm which is fitted to the discharge Vessel in which the cathode is housed. This, however, presents the disadvantage that the shape of the discharge vessel becomes more involved and the starting up voltage and the Voltage drop in the positive phase are unduly increased.

' In view of this fact tubes have also been developed in which the cathode and the anode are housed in a common vessel without anode arms and in this case the freedom from back stroke aimed at is endeavoured to be ensured by the interposition of screens only. As soon, however, as higher operating voltages, for example of more than 100 volts effective anode supply voltage with a rectified current of more than 10 amps. were proceeded with it was found impossible to obtain the freedom from back-arcing aimed at at an operating pressure of the tube of 0.3 mm. or even more. It would therefore have been necessary to tolerate the shorter life of the tube which is consequent upon a lower pressur at which the gas filling is sooner bound by the disintegrated material of the electrodes. It has thus always been necessary in the industry tolook for a compromise between freedom from back-fire on the one hand and life on the other hand and it was impossible to realise both properties at a time in the mannerv here desired.

The invention has for its object to ensure that current converting tubes which are suitable for the loads hereinbefore mentioned or even higher loads are free from back-fire under all conditions so that in view of the chosen pressure of more than 0.3 mm. mercury a fair length of life is ensured at any time.

According to the invention it is found possible to achieve this object by surrounding both the cathode and the anode which are housed in a common Vessel without anode arms by separate screens and by requiring as a criterion for the manner in which and the extent to whichsuch screen constructions have to be used that back strokes should not be allowed to occur at any cathode temperature, lower on the average than the operating temperature of the cathode.

Screens surrounding anode or cathode of the tube are known per se but only the use of the criterion of the invention has been found to permit of constructing the screening of both the anode and the cathode in such manner that the reliability of service and length of life of the tube aimed at are not achieved at the expense of other properties of the tube, such for example as low losses in the discharge path and simple construction of the discharge vessel.

A particularly suitable operating condition for the use of the criterion of the invention is constituted by the heating-up period of the cathode. Assuming the tube to be in normal use a reduction in the filament supply voltage, particularly if brought about slowly, does not always result in a particular tendency to back-stroke, because in this case a condition may ensue under which a smaller part of the cathode evolves the emission at a higher temperature. If, however, the tube is switched into a circuit in which at full anode voltage the full operating current will occur as soon as the cathode delivers the emission current required therefor conditions appear during the heating-up period under which the tendency to back-fire increases to a marked extent.

Inthis case, the temperature range between ordinary and operating temperature is traversed slowly, a zone being passed through in which the cathode readily sputters and thus is apt to initiate a back-fire. A highly sensitive test is obtained by even retarding this heating up, for example by applying a filament supply voltage of 50% of the normal value and connecting the tube into circuit at a normal anode voltage and current limitation. The period in which sputtering may occur is thus traversed particularly slowly.

In this case, the screens surrounding cathode and anode may advantageously be constructed and arranged in such manner that the speed at which the particles proceeding from the cathode f3 leave the cathode does not contain a com ponent in a direction towards an anode, even after one reflection against an internal part of the tube, the screening of the anode being on the other hand constructed and arranged so as to ensure that any particles that may proceed from the cathode or discharge cannot reach the anode.

In the particular case of the anode screen it is advantageous to provide the anode, except for the active surface situated at the front side, with a screening, preferablyconductive, which complete- 1y screens both the inactive surface and the pole wire of the anode, the union between pole screening and surrounding tube wall being formed Without any jars, for example by sealing.

In carrying into practice the idea of the invention it is the task of the constructor to construct the screening surrounding the anode and the cathode in such manner and to render the extent of such screening just so effective that there is always freedom from back fire at abnormally lowcathode temperatures.

In some cases a tube thus rendered freefrom back-fire may present the disadvantage that irregularities in the starting-up voltage of the main discharge in the positive phase occur due to the fact that there is no suificient ionisation of the entire main discharge path at the moment of; starting of the discharge in'the positive phase, particularly in those parts which are immediately adjacent the anode and are screened by the cathode screen in a highly efiective manner from the active surface of the cathode.

It is possible to obviate such. irregularities by forming one or more apertures on the side of the cathode screen adjacent the anode, independently of the pass-apertures for the main discharge, in such manner that even prior to the ionisation of the main dischargepath the electrons that are emitted by the cathode can reach the-immediate proximity of the anode, the prob.- ability that particles of material, for example of emitting material, hurled away from the cathode may reach the anodethrough the auxiliary apertures being practically zero, the latter only having a free section which is immaterial compared with the apertures that transmit the main discharge, preferably less than thereof.

In order that the invention may be clearly understood and readily carried into effect it will now be described more fully. with reference to the accompanying drawing inwhich forms of construction according to the invention are illustrated.

Figs. 1 and 2 are sectional views of current converting tubes according to the invention constructed respectively in metal and with one anode and in glass and with two anodes.

The tube shown in Fig. 1 has'a discharge vessel comprising a substantially cylindrical metal part I to which glass parts-2 and 3 are sealed so as to enable a vacuum to be obtained; The glass part 3 comprises two re-entrant parts 22 having sealed, to it: small metal hoods. 4- :for example of ferrochrome, which'in turn support the pole wires 5 of 9. directly heated incandescent cathode 6, supply leads i being welded to the hoods- 4 on the outside. The glass part 2. similarly carries a pole wire 8 for the anoded-which pole'wire is pr vided with a binding post In. The pole wires 5" and 8 are surrounded by screening tubes. H and l2-respectively of ceramic. insulating material, metalprotecting sleeves l3 and I4 protecting the glass parts of the tube from mechanical damage. The glass part 2 isunited, by sealing, without any jar to the ceramic tube l2 so that the discharge has no opportunity of pressing through along this path to the pole wire 8 of the anode 9. It is particularly convenientto keep the space at the back of the anode free from ionisation; this purpose is served by a labyrinth which comprises a circular plate [5 and a diaphragm it with the result that the space at the back of the anode is efiectively screened from the discharge of the tube. The incandescent cathode 6 and the anode 9 are separated by a further labyrinth which is constituted by a diaphragm l1 and an ovalshaped disc ill, the latter being arranged with its largest lengthalong the axis of the incandescent cathode 6, The discharge is thus forced to follow a curved path between the said electrodes and there is such a screening between them in a straight line that backfires are largely rendered difiicult. The manner in which in the tube described the anode is screened both from the surrounding dischargespace and from the oathode is so effective-that the tube satisfies the criterion of the invention, i. e., freedom from back-fire at low cathode temperatures. Under certain conditions it may be convenient, in order to ensure awell reproducible startingeup voltage in the pass-direction; to form a startingeup aperture ill in the disc H! which affords an oppor tunity for electrons proceeding from the cathode 6 to ionise the proximity of the anode to asuificient extent. Conversely, the aperture i9 has no injurious efiect on the blocking capacity of the tube since the probability that material particles pass through it is extremely low. At the r bottom of the tube there is liquid mercury 23 which is screened by the disc ZU-fromthe cathode.

In addition, the discharge vessel contains an.

argon filling of 0. 4 mm. of mercury pressure.

In contradistinction to the tube shown in Fig. 1 the tube shown in Fig. 2 has a glass discharge vessel 3| in which a cathode 32 and-two anodes 33 are similarly arranged by means of pole wires 34 and 35 which by means of small ferroch-rome hoods 36 and 3-1 are sealed'in the ends of the. tube so as to ensure a vacuum to be obtained. In

this case-alsothe anode is surrounded by a screen.

38 partially made of Wire gauze which completelyscreens the inactive part of theanode and leaves free only the active front side of the anode for the discharge and which by means of a. plate 39' is suspended from the ceramic pole-screening tube 40 which at 4| is sealed to the glass of the discharge vessel. The anodes are in addition separated by a fiat screen 42 which projects in.

front of the front-side ofthe anodes. The cathode is surrounded by a cylindrical cathode screen 44 which on the underside is closed by a base-plate 45 and is secured to the pole-screening tubes 450i the cathode, whilst. the aperture of the cathode screen is screened in the. direction of the anodes by a circular plate 41' having an arcuate edge 48. which leaves free an annular slit through which the discharge between cathode and anode is allowed to emerge, whereas par-v ticles proceeding from the cathode can only emerge'in a direction remote fromthe anodeand even afterreflection on the wall of the dis charge vessel retain, a direction which is remote from the anodes 33. The construction of'this tube renders it. desirable to form starting-up apertures 49* in the plate 4:! in: order to ensure reliable starting-up in the positive: phase. The, tube is filled with argon at a pressure of 0.7 mm. of mercury and in addition contains liquid mercury'zil! atithexbottom'. Under certain conditions.

it may be convenient to apply suitable potentials to the various above-mentioned screens. This tube also meets the requirement of the invention that at an abnormally low cathode temperature a back stroke does not occur and practice has shown that when the tube is in continuous use the freedom from back stroke is conserved under all conditions. Whereas with tubes of known screen construction the cathode, after an operating time of 1000 running-hours, used to lose its quality appreciably by disintegration or by poisoning by anode material, experiments conducted with tubes according to the invention revealed that the activated surface of the cathode had practically not changed even after normal operation for 2500 hours.

What I claim is: r

1. A rectifier tube, particularly for service in circuits involving potentials in excess of 100 volts and currents in excess of amperes, comprising an envelope and within said envelope a gaseous filling at a pressure in excess of 0.3 millimeter of mercury, an oxide cathode, an anode having an active surface and an inactive surface, a first screening member surrounding the inactive surface of said anode, and a second screening mem ber interposed between said cathode and the active surface of said anode, said screening members being so cooperatively arranged with respect to said cathode and said anode that electrons emitted from said cathode are prevented from directly passing to the active surface of said anode thereby preventing arc-back at lower than normal cathode temperatures.

2. A rectifier tube, particularly for service in circuits involving potentials in excess of 100 volts and currents in excess of 10 amperes, comprising an envelope and within said envelope a gaseous filling at a pressure in excess of 0.3 millimeter of mercury, an oxide cathode, an anode having an active surface and an inactive surface, a screening member surrounding the inactive surface of said anode, and a second screening member interposed between said cathode and the active surface of said anode, said screening members being so cooperatively arranged that arc-back is prevented upon application of normal anode voltage at normal anode currents at filament voltages in excess of onehalf of normal filament voltage.

3. A rectifier tube, particularly for service in circuits involving potentials in excess of volts and currents in excess of 10 amperes, comprising an envelope and within said envelope a gaseous filling at a pressure in excess of 0.3 millimeter of mercury, an oxide cathode, an anode having an active surface and an inactive surface, a first screening member surrounding the inactive surface of said anode, and a second screening member interposed between said cathode and the active surface of said anode, said screening members being so cooperatively arranged with respect to said cathode and said anode that electrons emitted from said cathode are prevented from directly passing to the active surface of said anode and from passing to the active surface of said anode after one reflection from any internal surface of said tube.

4. A rectifier tube, particularly for service in circuits involving potentials in excess of 100 volts and currents in excess of 10 amperes, comprising an envelope and within said envelope a gaseous filling at a pressure in excess of .3 millimeter of mercury, an oxide cathode, an anode having an active surface and an inactive surface, a screening member surrounding the inactive surface of said anode, and a second screening member interposed between said cathode and the active surface of said anode, said second screening member having an aperture therein arranged to permit electrons emitted by said cathode to pass to the vicinity of the active surface of said anode and to prevent said electrons from passing directly to the active surface of said anode thereby initiating ionization of said gaseous filling in the vicinity of said anode and preventing arc-back at lower than normal cathode temperatures.

JOHANNES GIJSBERTUS WILHELM MULDER.

No references cited. 

